Digital to analog converters (DAC) are used at the end of a digital processing chain where analog signal reproduction is required. Such uses include digital audio devices such as Compact Disc/MP3 players, HD radio and digital telephones. In all these applications, it is highly desirable to achieve a high signal to noise ratio (SNR) at the analog output while achieving the required, maximum high output signal level. In DAC applications, SNR is defined as the ratio between RMS of maximum amplitude output divided by the RMS of the integrated noise floor with no signal present and integrated within the frequency band of interest. SNR is usually specified in decibels (dB). For example, specifications for SNR of greater than 100 dB is common in smartphones and digital telephones and with modern sensitive headphones, a requirement for 120 dB is now considered state-of the-art.
Electronic noise is inherent in all analog circuits. Higher supply power (supply voltage and/or quiescent current) is traditionally used to lower noise floor and increase SNR. In mobile applications however, battery capacity limits available supply power and power consumption must be minimized.
The analog output of a DAC may be a voltage or a current. Most modern current-output DACs usually have differential outputs in order to achieve high common-mode and power-supply rejection and to reduce even-order distortion. An example of a DAC with differential current outputs and an operational amplifier differential to single ended converter is shown in FIG. 1A. An operational amplifier 203 connected as a differential to single-ended converter is often used to obtain a single ended voltage output across the desired frequency range. The amplifier 203 acts as a transimpedance amplifier. In this example, the output voltage, Vout, has a value of Vout=(Iout+−Iout−)×R, where Iout+ and Iout− are the complementary current outputs from the DAC 201 and R is the value in ohms of the feedback resistor FR 202 connected output and the inverting input of the operational amplifier and also for the shunt resistor SR 204 (of value R) that is connected from the non-inverting input of the operational amplifier to ground, as shown in the FIG. 1A below.
The transimpedance Vout/(Iout+−Iout−) of this connection is equal to R ohms, in other words Vout=(Iout+−Iout−)×R=2×Iout+×R. Also it should be noted that for this example, the voltage at both of the inputs of the operational amplifier is Vin_=Vin+=Iout+×R volts. The voltage gain of the amplifier, Av=Vout/Vin, is therefore equal to two.
There are many variations of transimpedance amplifiers but for each the prime parameters of interest are the transimpedance and the voltage gain.
One common alternative to the single ended amplifier transimpedance stage is a Differential-Input-to-Differential-Output “DIDO” amplifier acting as a differential output transimpedance stage which is in some cases followed by a second differential-to-single-ended voltage amplifier.
Another alternative to the current-output DAC with a transimpedance stage is a voltage-output DAC followed by a voltage amplifier or buffer.
Additionally, in some applications, the output may be current and not voltage.
There are three major sources of output voltage noise in a circuit comprising of a DAC with current outputs followed by a transimpedance amplifier. First the current noise, present at the output of the DAC, is converted to voltage noise at the output of the transimpedance amplifier. Second there is the resistor noise, thermal and flicker, which is related to the value of the transimpedance. Third there is the amplifier input referred noise that is created primarily in the input stages of the amplifier and which is not related to the value of the transimpedance. All three noise sources mentioned are further amplified to the output by the closed loop voltage gain of the amplifier.
There are a multitude of amplifier and DAC designs and the relevant design criteria are determined by the requirements of maximum output voltage range, maximum voltage range of the amplifier inputs and of the DAC outputs, and current output range of DAC, and in each case the design will tend towards a trade-off between the supply power and output noise when meeting the maximum amplitude requirement.
In practice, the noise floor is more apparent when the output signal is low, but the design for the transimpedance amplifier is ultimately set by the required gain and highest output signal that is required. In the example, the transimpedance is R, the output Vout=(Iout+−Iout−)×R, and the input voltage to the op amp is Iout+×R. The value for the transimpedance is set by the highest required output given the maximum Iout from the DAC, which in turn determines the value for the noise due to the transimpedance. As previously stated, the lower the value for the transimpedance, or R as depicted in the example, and the lower the voltage gain, the lower is the noise floor. The value for the transimpedance, or R in the example, must be set high enough such that the gain is sufficient to produce an output signal at a level to meet the highest output requirement for the application. This results in a practical limit to the noise floor when limited by DAC and amplifier power consumption.
Another consideration is that the signal voltage range at the input to the operational amplifier needs to be such that the amplifier input stage can safely operate without introducing distortion. In many cases the DAC architecture also imposes similar limits on its output voltage range. Therefore, in order to keep the signal voltage at the input of the transimpedance amplifier within limits set by the architecture and supply voltages, while supporting large output signals, Vout, it may be necessary to have a higher voltage gain across the amplifier. The problem is that increased voltage gain also amplifies the noise at the output of the amplifier. For example, the circuit shown in FIG. 1B is again an example of a DAC 201 with differential current outputs and an operational amplifier 203 differential to single ended converter, but in this case there is a higher voltage gain and hence a lower signal at the input of the operational amplifier for the same output signal voltage. The positive input of operational amplifier 203 is shunted to ground via shunt resistor SR 204 having an impedance of R/2 while the negative input of operational amplifier 203 is shunted to ground via shunt resistor SR 205 having an impedance of R while the feedback resistor FR 202 also has an impedance of R.
In this second example, the output signal is the same as in the first example Vout=(Iout+−Iout−)×R, but the signal voltage at the inputs of the operational amplifier is now Vin=Iout+×R/2, half that of the previous example. Alternatively, the lower input signal allows the value of R to be increased and hence the maximum output signal to be raised while keeping the input signals to the operational amplifier at levels that do not require high supply voltages to the amplifier input stage and DAC output. The point to note is that in order to cater for the large output signal requirement, not only is the value of R increased and hence the transimpedance R increased and, in turn, the resistive noise level, but also that the amplifier now has increased voltage gain which in turn increases the noise floor at the output from all three major noise sources by 6 dB.
The need to meet the gain criteria using the transimpedance amplifier effectively sets a limit on the noise levels and the result is that the desired noise levels and hence the signal to noise ratio of the output signal for state-of-the-art applications and devices is difficult to meet.
In other cases of DAC architectures such as voltage-mode DACs, similar noise analysis can be performed and similar design issues limit SNR